1. Field of the Invention
This invention relates to mechanisms for coordinating the operation of circuit breakers so that only one of the circuit breakers can be closed at any given time. In particular, it relates to an interlock for circuit breakers each having a state indicator indicating the open/closed state of the main contacts of the breaker, and an auxiliary trip member which allows the circuit breaker to be tripped and held open. More particularly, it relates to such an interlock which incorporates a compliant member which allows overtravel of the components coupled to the state indicators without binding of the auxiliary trip members.
2. Background Information
There are a number of applications where the operation of a pair of electrical switches be coordinated such that only one of the switches can be in the on position at a time. One such application is the transfer switch which is used to provide power to a load from two alternative sources. For instance, in many cases, an auxiliary power source is provided as an alternative to a utility source. As these two sources are independent, it is imperative that the one source be disconnected from the load before the other source is connected to prevent interconnection of two sources with a random phase relationship. Typically, circuit breakers are used as the switches in transfer switches so that overload protection is also provided. However, electrical switches without overcurrent protection are also used in transfer switches.
Another situation in which the operation of electrical switches must be coordinated is in ac motor control circuits such as reversing controls where one switch is used to connect the motor to a source with one phase rotation for forward operation and another switch connects the motor with the opposite phase rotation for reverse operation. As in the case of the transfer switches, the switches for the motor control may or may not have overcurrent protection.
It is known to coordinate the operation of two such electrical switches by the use of mechanical interlocks. One type of interlock couples the handles of the two switches. Another type of interlock used particularly when the switches are circuit breakers, utilizes a plunger mounted in the switch housing of each switch which when actuated engages the switch operating mechanism to prevent the switch from closing. In one such interlock, the plungers are coupled to opposite ends of a walking beam so that when one switch is closed it pushes down on its plunger thereby pivoting the walking beam and raising the other plunger to block closing of the other switch. The circuit breaker which is held open lacks sufficient force to override the interlock and force the closed switch open. A related type of interlock described in U.S. Pat. No. 5,436,415, utilizes a pair of pivoted cam plates each coupled to the plunger of one of the circuit breakers and joined by a connecting rod for opposed action. With one of the switches closed, the cam plate associated with the other switch is rotated to an overtoggle position so that a force generated on the plunger of that open switch is directed through the pivot axis of the associated cam plate and therefore applies no force through the connector which would attempt to override the plunger on the closed switch.
Still another type of interlock for a pair of electrical switches is described in U.S. Pat. No. 4,286,242. In this interlock, a plunger actuated by the closing of the contact arm on one switch rotates a connecting rod which acts through another plunger to hold the latch lever of the trip mechanism in the other circuit breaker in the unlatched or tripped position so that the second circuit breaker cannot be closed. A similar arrangement engaging the contact arm of the second breaker rotates a second connecting rod which in turn holds the latch on the first circuit breaker in the unlatched or tripped position when the second breaker is closed.
In a variation of the latter type of interlock, a pivoted arm which follows the rotational position of the pole shaft on one power circuit breaker acts through a connecting rod to rotate another pivoted arm on the other end of the rod to actuate a trip lever on a second power circuit breaker. A similar mechanism holds the first power circuit breaker in the tripped condition when the second is in the closed condition. The connections between the rods and the pivot arms engaging the trip levers on the respective breakers allow these pivot arms to be rotated during alternative actuations of the trip lever without moving the associated rod. To this end, the rods extend through a swivel which slides relative to the rod during reverse rotation of the pivot arm. This mechanism, however, does not allow for overtravel of components.
While all of these interlock mechanisms prevent simultaneous closing of the two switches, the latter two types especially do not accommodate well to variations in tolerances or wear or for overtravel of components.
There is a need therefore for an improved interlock for coordinating the operation of a pair of electrical switches.
There is a particular need for such an improved interlock which can accommodate for variations in tolerances and wear.
There is an additional need for such an improved interlock which will accommodate overtravel of some of the components to assure reliable operation.
These needs and others are satisfied by the invention which is directed to an interlock for coordinating the operation of electric power switches, and particularly to such electric power switches which have a state indicator indicating the open and closed state of the switch, and an auxiliary trip member which can be moved from an unactuated to an actuated position to hold the associated switch in the open condition. The novel interlock includes indicator followers each following an associated state indicator on an associated electric power switch, and auxiliary trip member actuators each positioned adjacent an associated auxiliary trip member on an associated electric power switch. The interlock further includes elongated connecting members, first couplers each coupling one end of an associated elongated connecting member to an associated indicator follower on a respective one of the electric power switches, and second couplers each coupling the other end of the associated elongated connecting member to an associated auxiliary trip member actuator on the respective other electric power switch to actuate the auxiliary trip member on the other electric power switch to the actuated position when the state indicator on the respective one electric power switch moves to the closed position. Each of the first couplers comprises a compliant member allowing initial movement of the associated state indicator on the one electric power switch before movement of the associated elongated connecting member and allowing overtravel of the state indicator when the associated auxiliary trip member actuator reaches its limit of travel. In the preferred form of the invention, the compliant member is a helical compression spring.
Preferably, each of the indicator followers includes a follower member biased by a biasing spring against the associated state indicator and the first coupler comprises a slip coupling mounted on the follower member with one end of the elongated connecting member slideably extending through this slip coupling. In this arrangement the helical compression spring is captured between the slip coupling and the associated end of the elongated connecting member. In the most preferred arrangement, the state indicator pivots on the associated electric power switch and the follower member is also pivoted. In this arrangement the slip coupling comprises a swivel mounted on the follower member with the elongated connecting member sliding through the swivel. An adjustment mechanism permits adjustment of both the length of the connecting member and the preload on the helical compression spring.
Preferably, the second couplers comprise one way connectors which transmit motion of the elongated connecting member to the associated auxiliary trip member actuator but do not transmit movement of the auxiliary trip movement actuator to the elongated connecting member so that where the auxiliary trip member on the switch can be actuated by an alternate arrangement, this action is not transmitted back through the interlock.